As a discharge load using discharge energy, there are various laser devices, a discharge lamp lighting device, a strobe device, an electro-discharge machining device, a fusion splicing device of optic fiber, and a vacuum device for forming metallic thin films on the surfaces of various materials by discharging, and discharge loads are broadly used in various technical fields. These discharge loads often require constant power.
In a case of a discharge load power supply unit as described above, since the output voltage fluctuates in a certain range, depending on a plasma state, the constant power should be ensured in the range. For example, in a case in which the output is 800 W, the rated output voltage is 800 V, and output current that can be supplied at that time is 1 A. Then, when the output voltage is 400 V, it is required to supply the output current of 2 A to a discharge load. However, in a normal discharge load power supply unit, when designing a power supply with 800 V-1 A, the maximum current at 400 V is at most approximately 1 A (depending on its circuit mode). The output power of 800 W cannot be satisfied by either the constant output voltage of 800 V or the output voltage of 400 V, which is substantially lower than 800 V.
Among such power supply units, some are configured so that it is possible to output power of 400 V-2 A by providing a medium voltage tap to a secondary-winding wire of a transformer 6 to switch the output terminal to the medium voltage tap to produce low voltage/large current. However, for switching of the tap, a switching operation has to be done by opening a housing of the power supply unit, which is troublesome since it requires time and effort, while safety must be ensured by confirming the discharge of residual charge upon switching.
In order to avoid this, it is necessary to design a power supply with large capacity that can supply maximum current. For example, in the previous example, it is necessary to manufacture a power supply unit with power capacity of 800 V-2 A, i.e. a power supply device that can output 1.6 kW. In this case, since the power of a transformer is substantially proportional to the product of the maximum voltage and the maximum current, a transformer having a power capacity with 1.6 kW is needed, and the power of the transformer becomes approximately double the transfer power of 800 W. This is obviously not cost-effective since the entire power supply unit becomes larger and heavier, which leads to an increase in cost.
Inventions that solve the above problems have been proposed. For example, the invention disclosed in Japanese Unexamined Patent Application, Publication No. 2006-191766 (hereinafter referred to as Patent Document 1) realizes a multi-functional rectification circuit that can supply a substantially constant power by operating to be automatically mutually switched between a normal full-wave rectification circuit and a voltage doubler rectification circuit, depending on a requirement of load current. This automatic switching is performed electronically without necessitating a physical switching such as a mechanical switch and a semi-conductor switch, as well as switching of a medium voltage tap of the transformer. In addition, in a case of the load being a discharge load, it is possible to supply voltage that is approximately double the alternating voltage necessary to generate discharge using an extremely simple circuit configuration and a simple normal pulse-width control method of an inverter circuit, and when the discharge load becomes a discharge state and a low impedance, it is possible to supply a DC constant power required to maintain a steady discharge state having a voltage that is lower than the initial high voltage.
The DC power supply disclosed in Patent Document 1 is a series resonant converter circuit that causes resonant inductance and a resonant capacitor to resonate in series in order to improve power conversion efficiency. The resonant converter circuit can obtain a sinusoidal resonant current waveform. However, a case of using a resonant converter circuit as a DC power supply for a vacuum device has the following problem. Although the load power required for a vacuum device, i.e. a DC output power required in a resonant converter circuit, generally requires constant power, plasma voltage greatly changes depending on changes in the gassing conditions, target materials, etc. as described above. In this case as well, pulse width control is performed on a resonant converter circuit so as to make substantially constant output power.
Since a resonant converter circuit is generally designed for the operation with rated voltage, the resonant converter operating at the rated voltage outputs a sinusoidal resonant current. However, the resonant current waveform deviates from the sinusoidal waveform accompanying the gradual decline in the DC output current from the rated voltage. For example, in a case in which the output voltage is lower to approximately half the rated voltage, and the output current must be amplified up to approximately double the rated current, since the resonant current of the resonant converter becomes an AC waveform that deviates from the sinusoidal waveform according to the principles of resonance, the power efficiency of the resonant converter circuit declines and noise becomes large.